@oqhk1//6Lhmdq`knesgdLnmsg9Ogdm`jhsd This month’s mineral is a rare beryllium-silicate gemstone collected in Myanmar (formerly Burma) at a recently discovered deposit in the fabled Mogok Stone Tract, considered by many as the richest and most historic gemstone region on Earth. OGXRHB@K OQNODQSHDR Chemistry: Be2SiO4 Beryllium Silicate Class: Silicates Subclass: Nesosilicates (Independent Tetrahedral Silicates) Group: Phenakite Crystal System: Trigonal (Hexagonal) Crystal Habits: Usually in well-developed, flattened rhombohedrons, tabular-to-short prismatic crystals; less commonly as long prisms; lengthwise striations. Crystals usually less than one-half inch in size; penetration twins common. Also granular and as spherulites and columnar aggregates. Color: Usually colorless; also white and pale shades of yellow, yellowish-brown, and pink. Luster: Vitreous Transparency: Transparent to semi-transparent Streak: Colorless Cleavage: Poor in one direction Figure 1 Phenakite. Fracture: Conchoidal to uneven, brittle Hardness: 7.5-8.0 Specific Gravity: 3.0 Luminescence: None Refractive Index: 1.654-1.670 Distinctive Features and Tests: Best field marks are rhombohedral crystals, lengthwise striations, and hardness substantially exceeding that of quartz. Well-developed crystals occur only in pegmatites. Dana Classification Number: 51.1.1.1 M @L D The name of this month’s mineral has two accepted spellings. The preferred spelling, “phenakite,” currently used in all mineralogical literature, is pronounced FEH-na-kite; the alternative and somewhat outdated spelling is “phenacite,” which is pronounced FEH-na-site. The name is derived from the Greek phenakos, or “deceiver,” a reference to the previous confusion with quartz and tourmaline. Before its mineralogy and chemistry were understood, phenakite was known as “white tourmaline” and “white schorl.” In European mineralogical literature, phenakite is known as “phenacita,” “phenacit,” and “phenakit.” “Germanium phenakite” is a pinkish, germanium-containing variety. BNL ONRHSHNM Our 133rd featured mineral is our second containing the element beryllium, after the red beryl from Utah we featured in March 1999. Yes, we featured red beryl! Of course, they were small specimens, but the Deluxe pieces were in matrix. The mine there has been closed for years, and what is still available is extremely expensive. This is our second featured mineral from Myanmar, after the red spinel in white marble matrix we sent in September 2000. Myanmar certainly has the potential to produce other minerals for us to feature, as you will read, along with information on the fascinating element beryllium. Bnoxqhfgs 1//6 ax Qhbg`qc % Bgdqxk Rhsshmfdq L hmdq`k ne sgd L nmsg Bkt a 0 66/ N qu hkkd @ u dmt d B`l aqh`+ B@ 8 2 3 17 0 ,7 //,8 3 0 ,4 4 8 3 v v v -l hmdq`knesgdl nmsgbkt a-nqf 1 @oqhk1//6Lhmdq`knesgdLnmsg9Ogdm`jhsd Phenakite, chemical formula Be2SiO4, contains the elements beryllium (Be), silicon (Si), and oxygen (O). Its molecular weight is made up of 16.37 percent beryllium, 25.51 percent silicon, and 58.12 percent oxygen. The phenakite molecule has a positively charged cation and a negatively charged radical (a group of ions that acts as an entity in chemical reactions). The cation consists of two ions of beryllium (2Be2+) 4- with a collective +4 charge. The anion, the silicate radical (SiO4) , has a collective -4 charge. The balance of these cationic and anionic charges provides the phenakite molecule with electrical stability. Phenakite is a member of the silicates, the largest and most abundant class of minerals. Silicates are combinations of silicon and oxygen with one or more metals. The basic silicate structural unit is the silica 4- tetrahedron (SiO4) , which consists of a silicon ion surrounded by four equally spaced oxygen ions positioned at the four corners of a tetrahedron (a four-faced polyhedron). In silicate minerals, silica anions and metal cations join together like polymers (repeating chains) to form seven types of structures: independent tetrahedral silicates (nesosilicates); double tetrahedral silicates (sorosilicates); framework silicates (tectosilicates); single- and double-chain silicates (inosilicates); ring silicates (cyclosilicates); and sheet silicates (phyllosilicates). Phenakite is a nesosilicate, or independent tetrahedral silicate. In nesosilicates, metal cations are packed tightly between silica tetrahedra that have no direct bonding to other tetrahedra. In the rigid nesosilicate crystal lattices, each silica tetrahedron is bonded covalently and ionically to the metal ions. Because of close atomic packing and strong covalent bonding, nesosilicates are hard, relatively dense, and form short, blocky, somewhat square crystals with little cleavage. Due to these properties, more gemstones (topaz, zircon, peridot, all garnets) come from the nesoslicates subclass than any other silicates subclass. The phenakite lattice is a repeating, three-dimensional structure with nearly equal bonding in all directions. Because strong covalent bonding predominates, phenakite has poor one-directional cleavage. Its hardness of Mohs 7.5-8.0 is considerably greater than that of quartz [silicon dioxide, SiO2, Mohs 7.0] and approaches that of topaz [basic aluminum fluorosilicate, Al2SiO4(F,OH)2, Mohs 8.0]. Phenakite’s hardness is somewhat directional, being hardest along the two planes with predominant covalent bonding. Although phenakite’s constituent elements all have light atomic weights, close atomic packing gives the mineral considerable density as shown by its surprisingly high specific gravity of 3.0. Because surrounding oxygen ions completely shield the two beryllium ions, phenakite exhibits no metallic properties. Beryllium, one of the alkaline-earth elements, is a gray, brittle metal that ranks 51st in crustal abundance, about the same as tin. Beryllium has a very low atomic weight of 9.012, only three-quarters that of carbon, and is the lightest of all stable metals. Beryllium has electrical applications and is a component of lightweight, exotic alloys. With its unusual ability to transmit X rays and absorb neutrons, beryllium has many applications in radiology and nuclear-power generation. As an allochromatic mineral, pure phenakite is colorless. But trace amounts of nonessential elements such as iron and aluminum alter the light-absorption characteristics of the lattice to create a range of pale colors. A white color in phenakite is due to traces of nonessential elements that disrupt the crystal-lattice symmetry in a manner that increases the reflectance of white light. Phenakite crystallizes in the trigonal system, which has four axes of symmetry. Three are of equal length and separated by angles of other than 120 degrees; the fourth axis is shorter and perpendicular to the others. The trigonal system is sometimes considered part of the hexagonal system, which also has four axes, three of equal length on a common plane and separated by angles of 120 degrees. Although trigonal minerals sometimes form prisms and dipyramids typical of the hexagonal system, their general symmetry is rhombohedral. Bnoxqhfgs 1//6 ax Qhbg`qc % Bgdqxk Rhsshmfdq L hmdq`k ne sgd L nmsg Bkt a 0 66/ N qu hkkd @ u dmt d B`l aqh`+ B@ 8 2 3 17 0 ,7 //,8 3 0 ,4 4 8 3 v v v -l hmdq`knesgdl nmsgbkt a-nqf 2 @oqhk1//6Lhmdq`knesgdLnmsg9Ogdm`jhsd Phenakite’s rhombohedral structure and relatively high density make it a good refractor of light. This ability to bend light is measured by the index of refraction (R.I.), which is the ratio between the speed of light in air and in a crystal. Higher numerical values indicate greater degrees of refraction. High indices of refraction in mineral crystals correspond directly to brightness and brilliance in gems. Phenakite’s index of refraction of 1.654-1.670 is higher even than that of such gemstones as beryl (emerald) and topaz. Phenakite forms in three distinct mineralogical environments: hypothermal veins (hydrothermal mineralization emplaced at high temperature) in association with topaz [basic aluminum fluorosilicate, Al2SiO4(F,OH)2] and cassiterite [tin oxide, SnO2]; granite pegmatites with quartz [silicon dioxide, SiO2], microcline [potassium aluminum silicate, KAlSi3O8], and albite [sodium aluminum silicate, NaAlSi3O8]; and regional metamorphic rocks with emerald, the green gem variety of beryl [beryllium aluminum silicate, Be3Al2Si6O18], and chrysoberyl [beryllium aluminum oxide, BeAl2O4]. The Dana mineral-classification number 51.1.1.1 identifies phenakite as a nesosilicate with insular (isolated) silica groups (51). The subclassification (1) next specifies that its silica groups have “four- 4- oxygen coordination” [(SiO4) ]. Finally, phenakite is a member of the phenakite group (1), as the first (1) of three members. This group also includes willemite [Zn2SiO4], the well-known fluorescent mineral found at the famous Franklin and Sterling Hill mines in New Jersey, and eucryptite [LiAlSiO4]. BNKKDBSHM F KNB@KHSHDR A number of worldwide localities exist for phenakite, but few produce in quantity. Our phenakite specimens were collected at the Pelelni pegmatite mine near Khetchel village in the Molo quarter of Momeik Township in the Shan state of Myanmar. Other phenakite sources in Asia include the type locality at the Malyshevskoye emerald and chrysoberyl mines on the Tokovaya River at Ekaterinburg, Sverdlovskaya Oblast’ in the Urals Region of Russia; and the Koktokay pegmatite field at Fuyun in Altay Prefecture of the Xinxiang Autonomous Region of China. In Africa, phenakite occurs